1. Field of the Invention
The present invention relates to computer graphics systems, and more specifically to a method and apparatus for sampling an analog display signal received in a digital display unit.
2. Related Art
Digital display units (e.g., flat panel monitors) are often used to display images encoded in an analog display signal. An analog display signal generally includes display data signal (e.g., in RGB format) and corresponding synchronization signals (e.g., HSYNC and VSYNC). The display data signal identifies a color intensity for each point of an image and the synchronization signals provide a time reference such that each portion of the display data signal can be correlated with a corresponding portion of the image.
Display signals are typically generated by a digital-to-analog converter (DAC) in a graphics source generally located outside of a digital display unit. The DAC generates a portion of a display data signal by processing each of several pixel data element values representing an image. Each pixel data element value generally represents the color intensity of a point of the image such that the display data signal represents the overall image. An analog-to-digital converter (ADC) located in a digital display unit usually samples display signals to recover the pixel data elements and display the represented images.
The analog display signals received by a digital display unit are characterized by frequency components. In general, signals which can change amplitude levels quickly (or sharply) include high bandwidth components, and are accordingly termed as wide bandwidth signals. On the other hand, signals which can only change slowly as a function of time may be termed as narrow-band signals.
In general, it is desirable that analog display signals be wide bandwidth signals as display signals may represent adjacent points differing substantially (e.g., no color intensity to complete brightness) in intensity level and an ideal display signals needs to transition in amplitude instantaneously to represent such adjacent points. Such an ideal signal (hereafter "stairway signal") resembles a "stairway" as is (illustrated below with reference to FIG. 1A) well-known in the relevant arts.
Unfortunately, the input to an ADC in a digital display unit may not resemble an ideal stairway-signal for several reasons. For example, DACs in typical graphics sources may not be able to generate stairway shaped signals due to practical limitations in cost and manufacturing technologies. As a result, overshoots of some magnitude are typical with transitions as illustrated with reference to FIG. 1A, which depicts a graph of a portion of display signal with amplitude on Y-Axis and time on X-Axis. The ideal stairway signal is shown as line 150.
Time duration 170 represents the pixel period, which is a time duration when the signal level represents the pixel data element value from which the portion is generated. Time duration 110 represents a rise time as the analog signal level changes to a desired amplitude level represented by stairway signal 150. Because of ADC imperfections, overshoot 120 is typically present in wide bandwidth analog display signals. The overshoot decreases substantially over time, and the overshoots are down to a large degree in the time duration 160. Therefore, time duration 160 ("sampling duration") is a suitable period for taking samples representing the corresponding pixel data element.
Unfortunately, overshoots (or other deviation from the ideal stairway signal) exist to some degree even in sampling duration 160, which may cause the sampled data element value to deviate from the corresponding pixel data element value. The deviation can be exacerbated with any other distortions introduced in a received analog display signal. For example, additional noise may be introduced in a cable carrying the display signal from a graphics source to a display unit. In addition, reflections may also be introduced due to impedance mis-match at various connections in the path of a display signal from a DAC to an ADC. Ringings of the ADC may introduce more distortions as is also well known in the relevant arts. Distortions may be problematic because the result may be an increase in the deviations from the correct value represented by stairway signal 150.
Due to the contribution of such distortions including the persistent overshoots, the sampled data element values can vary from the correct value represented by the ideal stairway signal 150. The effect is that perceptible display artifacts may be present in the displayed images. For example, the artifacts may have the form of rolling diagonal bars or horizontal lines or low frequency flicker.
The phenomenon causing the artifacts can also be appreciated based on the spectral analysis of typical wide bandwidth-band analog display signals. According to the Nyquist Theorem well known in the relevant arts, a signal having a bandwidth of B needs to be sampled at or greater than 2.times.B samples per second for accurate representation of the signals. Therefore, if a signal has a bandwidth of 50 Mega Hertz, the signal needs to be sampled at 100 Mega samples/second. However, as noted above, analog display signals are typically characterized by much higher bandwidth components. When sampled at a lower sampling rate (equaling a lower frequency at which the pixel data elements are encoded in the analog display signals), the sampled values include aliased components associated with the under sampled frequencies.
Display artifacts result due to such aliased components. These type of display artifacts may be undesirable in some situations.
To minimize the artifacts, one may employ one or more of a slower DAC, a low pass filter at the output of a fast DAC (before the analog signal is passed on to a cable connecting to a display unit), limited analog bandwidth ADC, and a low pass filter before the ADC in a digital display unit. An analog display signal presented to an ADC as a result of any such schemes is a signal which responds slowly to changes in the values of adjacent pixel data elements as generally illustrated with reference to FIG. 1B.
FIG. 1B illustrates a portion of an analog display signal (corresponding to a pixel data element) generated either as an output of a low pass filter or a slow DAC. Similar time durations are represented by similar numerals in comparison to FIG. 1A. The overshoots and external noise are substantially eliminated. However, due to the jitter generally present in sampling clocks driving ADCs and the short sampling period 160 particularly with the analog display signals generated at high dot clock speeds, the sampled data values may differ substantially from the corresponding pixel data element value corresponding to the display signal portion.
The result may be a decrease in the effective number of quantization levels in the operation of ADCs, and thus a reduction of effective number of bits (ENOB) as is well known in the relevant arts. As an illustration, assuming an ADC can generate an 8-bit output with the ability to discriminate between 2.sup.8 (256) color intensities (or quantization levels), only 2.sup.5 (ENOB=5) different values may be generated from an ADC because of the narrow bandwidth input signal to an ADC. Smaller ENOBs usually means a degradation in the display quality, and thus undesirable.
What is therefore needed is an effective method and apparatus which minimizes the display artifacts caused by noise components when displaying images encoded in wide-band analog display signals.